Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals and thus reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in publications by C. H. Bennett et al., entitled “Experimental Quantum Cryptography,” J. Cryptology 5: 3-28 (1992), and by C. H. Bennett, entitled “Quantum Cryptography Using Any Two Non-Orthogonal States,” Phys. Rev. Lett. 68 2121 (1992).
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. During the QKD process, Alice uses a random number generator (RNG) to generate a random bit for the basis (“basis bit”) and a random bit for the key (“key bit”) to create a qubit (e.g., using polarization or phase encoding) and sends this qubit to Bob.
The above mentioned publications by Bennett each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The interferometers need to be actively stabilized to within a portion of quantum signal wavelength during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent) discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
However, in autocompensated and actively stabilized QKD systems, it is the optics layer that is stabilized or compensated. As it turns out, drifts can and do occur in the electronics necessary to stably operate the QKD system. For example, in a phase-encoding QKD system, if the voltage used to set the phase modulators drifts over time, then the phase imparted to the optical pulses will drift over time. The same is true for polarization modulators in polarization-encoding systems. This drift results in the pulses not having precise phase or polarization modulation, which reduces the ability to detect the encoded pulses. If this drift goes uncompensated, the operation of the QKD system continually diminishes, and can even reach the point where the QKD system can no longer operate.
Also, when performing the analysis of the basis measurements under particular QKD protocol (e.g., the BB84 protocol), there needs to be a 50:50 chance of Bob's detectors detecting signals measured in a basis different from Alice's basis. To the extent this probability differs from 50:50, an eavesdropper has a potential advantage because the uncertainty associated with a “wrong” basis measurement is reduced. This variation from a 50:50 probability distribution can occur because the modulator basis voltages are not “orthogonal,” i.e., a change in basis voltage by a discrete amount (e.g., from V[−π/4] to V[π/4]) does not result in the modulator providing the corresponding phase difference of π/2.